1. Field of the Invention:
The present invention relates to a hetero junction bipolar transistor (HBT), and more particularly, it relates to a highly efficient HBT in which a fine emitter is provided.
2. Description of the Prior Art:
With the development of an HBT in recent years, there has arisen a demand for decreasing the emitter size in order to realize high density in a so-called "emitter-up type" HBT. In the production of an HBT, a part of an emitter layer is removed by mesa etching, thereby exposing the corresponding part of a base layer. As a result of this mesa etching, the periphery of the emitter-base heterointerface is exposed outside. However with a decrease in the HBT size, the surface recombination current at the exposed periphery of the emitter-base heterointerface has become too large to be ignored. This has led to a decrease in the current amplification factor of the HBT.
Therefore, in order to prevent the decrease in the current amplification factor, the emitter-base heterointerface requires passivation. As a process for passivating the emitter-base heterointerface, there has been proposed the deposition of Na.sub.2 S.9H.sub.2 O on the HBT surface (see Applied Physics Letters, 52(3)218, 1988).
In the above-mentioned conventional passivating process, this exposed periphery of the emitter-base heterointerface is covered with Na.sub.2 S.9H.sub.2 O which is deposited on the entire surface of the HBT.
However, the HBT in which the exposed periphery of the emitter-base heterointerface has been passivated by Na.sub.2 S.9H.sub.2 O mentioned above has a number of drawbacks as follows: (1) the current amplification property of the HBT is susceptible to the moisture present in the atmosphere due to the high water absorbing power of Na.sub.2 S.9H.sub.2 O; (2) the thin Na.sub.2 S.9H.sub.2 O film has ionic conductivity, resulting in excess current; and (3) the current amplification factor can be maintained only for a short time in air due to the high absorbing power of Na.sub.2 S.9H.sub.2 O. These cause serious problems in the practical use thereof.